In 1984, International Standards Organization (ISO) proposed an Open Systems Interconnection Reference Model (OSI-Model) to define the communication standard between different computers, which enables computers to communicate with each other under a common protocol. The OSI-Model is a structured multilayer communication architecture. It uses 7 vertically-connected layers to define the functions. From the bottom up, they are physical layer (PHY), data-link layer (or MAC), network layer, transport layer, session layer, presentation layer and application layer. An interface exists between two adjacent layers to provide link and communication.
The physical layer is responsible for passing bits onto the physical medium (e.g. twisted pair, coaxial cable) connected to other receivers. On top of the physical layer is data-link layer, which is used for segmenting data into several data frames in order to apply flow control. Data-link layer attaches an error detection and recovery header to every data, and then passes the data onto physical layer for transmission. There exists an interface, which includes a medium access control device (MAC device) and a physical layer signal control device (PHY apparatus) between the physical layer and data-link layer.
The Institute of Electronic and Electrical Engineers (IEEE) has worked out the 802.3u standards, which defines the communication protocols between the PHY apparatus and the MAC device. When data are transmitted between the MAC and the PHY apparatus, the data must conform with a specific data frame format. Refer to FIG. 1 which depicts the structure of the data frame format 10 under the IEEE 820.3u definition. The data frame format 10 contains 3 segments: (1) MAC header 11, (2) MAC data 12, and (3) error checking field 13.
The MAC header 11 further includes a destination address 111, a source address 112 and a sequential number field 113. The destination address 111 contains 6 bytes, and the destination address 111 is used as a broadcast address if all 6 bytes are represented by FF in hexadecimal. If destination address 111 is a multicast address, the first bit of the 6 bytes is designated as 1, otherwise it is set to 0 to show that the destination address 111 is an unicast address, or a physical address dedicated to one LAN adapter. The source address 112 also contains 6 bytes to represent the address of sender, and generally the source address 112 begins with the first bit of 0, but the source address 112 can't be all-zero bytes. In other words, when data are transmitted between the PHY apparatus and the MAC device, the data frame format 10 can't have all-zero bytes in the source address 112.
IEEE 802.3u further defines a communication protocol for PHY apparatus of different Network Interface Cards (NICs) to let PHY apparatus at any ends to communicate with each other under the same data transfer rate or under same duplex mode. Data transfer rate can be either 100 Mbps or 10 Mbps, whereas duplex mode can be full-duplex or half-duplex. So there are 4 possible combinations of the transmission configuration between different NICs. PHY apparatus need to monitor and update the transmission configuration when a change detected in order to maintain consistent transmission configuration.
But a problem arises, as the IEEE 802.3u standard does not define how to inform the MAC device of the update transmission configuration of the PHY apparatus when the PHY apparatus does so. As the IEEE 802.3u standards does not define the scenario, it often happens that the MAC device does not simultaneously update its transmission configuration in accordance with the PHY apparatus. This results in communication error or lost of communication link. For instance, the PHY apparatus and the MAC device are operating at 100 Mbps and fill-duplex mode at the beginning, and then the external transmission configuration changes to 10 Mbps and half-duplex mode. Since the PHY apparatus can automatically sense the change, it soon adjusts to the new configuration without noticing the MAC device. So the MAC device won't be able to switch to the new configuration. Then the PHY apparatus and the MAC device are working under different transmission configuration, and an error easily occurs.
To solve the problem, some conventional MAC devices regularly monitor the PHY apparatus to see if there's an updated transmission configuration and adjust themselves accordingly. But there's a drawback by this mechanism. For instance, if the external transmission configuration stays the same for a long time, the MAC device still has to monitor the PHY apparatus every now and then, which results in a performance downgrade. The preferable solution is to let the PHY apparatus inform the MAC device when it detects a change. In this way, the hardware configuration of MAC device need not be changed and the communication efficiency won't be sacrificed.